1. Field of the Invention
The present invention relates to an optical communication system, and, more particularly, to an optical communication system capable of assigning an unused wavelength to a connection request from a network in accordance with the wavelengths being used in a wavelength-multiplex communication path. Such a system includes a wavelength-sharing optical exchange system which assigns, to a call, a wavelength suitable for a condition of the exchange network at the time of call and wavelength-multiplexes a plurality of calls so as to effectively utilize a wavelength band.
2. Description of the Prior Art
In an optical communication system according to the prior art which accommodates a plurality of terminals by using wavelength-multiplex communication paths, different wavelengths inherent to the respective terminals are assigned thereto for transmission or reception, so that signals having the same wavelength may not be transmitted through the same wavelength-multiplex communication path. For example, FIG. 1 is a diagram illustrating a conventional optical star-type communication network. In FIG. 1, the reference numeral 201 designates a terminal; 202 a signal source; 203 a transmission light source; 204 a photocoupler; and 205 a receiving circuit capable of selecting a wavelength. FIG. 2 is a diagram illustrating a conventional optical concentrating system using wavelength-multiplex communication paths. In FIG. 2, the reference numerals 201-1 through 201-n designate terminals; 206 a wavelength-multiplex communication path; 207 a network accommodating wavelength-multiplex communication paths 206; and 208 an interface for interconnecting communication path 206 and network 207.
In operation, a wavelength .lambda.1 inherent to terminal 201-1 is assigned to transmission light source 203 contained in terminal 201-1. When a signal is transmitted to terminal 201-1 from any other terminal, a signal having wavelength .lambda.1 is transmitted to terminal 201-1 via photocoupler 204 and then receiving circuit 205 in terminal 201-1 selects the signal having wavelength .lambda.1 and receives it. Upon transmission, each terminal decides whether an optical signal having the same wavelength as that of the signal to be transmitted exists on the communication path. If there is no such optical signal on the communication path, that terminal starts transmission of the signal. In FIG. 2, wavelengths .lambda.1 through .lambda.n for transmission are fixedly assigned to transmission light source 203 in the respective terminals 201-1 through 201-n, such that the wavelengths are not overlapped. Optical signals transmitted from the respective terminals are multiplexed on wavelength-multiplex communication path 206 via photocouplers 204 and coupled to network 207.
As explained above, in a method of fixedly assigning to terminals reception wavelengths and transmission wavelengths inherent to the respective terminals, the number of terminals is limited by the wavelengths because each wavelength corresponds to one particular terminal. Even if the wavelength assigned to one terminal is not in use, this wavelength cannot be used by other terminals, resulting in inefficient utilization of wavelength.
FIG. 3 illustrates a space-sharing optical exchange system as disclosed in the Japanese Patent Public Disclosure No. 48895/87. In FIG. 3, the reference numerals 301 through 304 designate terminals; 305 an optical switchboard; 341 through 344 subscriber optical fibers; and 351 through 354 optical switches. Respective terminals 301 through 304 each include two light sources having wavelengths .lambda.1 and .lambda.2 and select either one of the wavelengths for transmission.
In operation, switchboard 305 directly interconnects optical switches 351 and 354. Each of terminals 301 through 304 includes two light sources having wavelengths of .lambda.1 and .lambda.2 as explained above and selects an optical signal having either one of the wavelengths for transmission. For example, if communication is required between terminal 301 and terminal 304, terminal 301 transmits a signal having wavelength .lambda.1 while terminal 304 transmits a signal having wavelength .lambda.2. At the same time, switchboard 305 establishes a communication path running from subscriber fiber 341 through optical switches 351 and 354 to subscriber fiber 344. In this way, terminals 301 and 304 may be interconnected. Before communication between two terminals is commenced, transmitting and receiving wavelengths to be used by each terminal are instructed by a subscriber signal from a control circuit of optical switchboard 305 to a control circuit of the respective terminals.
As explained above, according to a system of assigning a wavelength to a call, respective terminals and an optical switchboard are interconnected in a star form, requiring a high cost of concentration. Besides, since wavelength-multiplexing is merely available for upward and downward signals in a wavelength-multiplex communication path, a transmission band contained in a wavelength range is not effectively utilized. Since a network capacity is limited by a spatial switch, any increase in capacity is difficult due to the structure of devices used. If terminals are further added later on, the switchboard has to be modified.